1. Technical Field
This invention relates to re-imaging optical systems and, more particularly, to a catatrioptric re-imaging optical system utilizing reflective, refractive, and diffractive optical elements. 2. Discussion
Re-imaging optical systems generally include two optical modules, an objective group and a relay group. The objective group precedes the intermediate image plane and the relay group follows the intermediate image. Re-imaging optical systems are key optical components in many optical sensors. Re-imaging optical systems also have unique properties. Some of their properties are better rejection of off axis radiation, one hundred percent cold shielding with the cold stop right next to the detector array, and an accessible entrance pupil.
Two examples of common re-imaging optical systems are all reflective three mirror anastigmats and catadioptric optical systems. Three mirror anastigmatic systems generally include two concave mirrors and one convex mirror. Therefore, two of the mirrors are positive and one is negative power. The advantage of a three mirror anastigmat is that it is of a simple optical design which has substantially no chromatic aberration and is generally, relatively speaking, less expensive to fabricate when compared to refractive optical systems. Unfortunately, it is difficult to avoid vignetting or central obscuration without jeopardizing the field of view coverage. The vignetting problem is particularly severe for the tertiary mirror. In order to clarify both the exit pupil and image planes from radiation reflected off the secondary mirror, before impinging on the tertiary mirror, either the line of sight is offset from the optical axis or the tertiary mirror is purposely tilted. Thus, the total usable field of view is limited.
While a catadioptric optical system experiences less vignetting and obscuration problems, they are generally more complicated. The complication is due to the requirement of chromatic aberration correction. To alleviate the chromatic aberration problem, the optical power of each refractive optical element is reduced. In many cases, the optical power of the refractive optical element group is insignificant as compared to that of the reflective optical element.
Thus, three mirror anastigmatic optical systems exhibit vignetting and observation problems, while catadioptric optical systems are complicated due to the requirements for chromatic aberration correction. The present invention provides an optical system which overcomes the above problems.